Yes, correct. N toxicity. Feed plain pH adjusted water just as you are. You have the right instincts and approach. I would not stress and cause more issues than just this, i.e., it is what it is at this point and you can only do so much with a TLO soil at the base. Are you using any bottled/liquid nutes on top of that soil? Or is this coming from the soil and nothing else? Could be your roots are hitting that bottom 1/3 which is probably a little hot and high in N.
Good luck